1. Field of the Invention
This invention relates to fuel cell separators, to methods for making the same, and to fuel cells using the separators. In particular, the invention relates to a fuel cell separator having a low electrical resistance in its thickness direction and a highly accurate size, to a method for making the fuel cell separator, and to a fuel cell having a high generation efficiency using the fuel cell separator.
2. Description of the Related Art
Fuel cells having high generation efficiency and producing fewer contaminants are anticipated as future power plants. Fuel cells are classified as: the alkaline type, the. phosphate type, the melted carbonate type, the solid electrolyte type, and the solid polymer type depending on the type of electrolyte used. The solid polymer electrolyte type can generate electric power at a low operational temperature and has attracted attention as a small-scale power plant for domestic use or as a portable power plant for electric cars.
As shown in a perspective view in FIG. 1, a solid polymer-type fuel cell includes a unit cell of a solid polymer electrolyte 1, an air electrode 2 and a fuel electrode 3 provided on a front face and a back face, respectively, of the solid polymer electrolyte 1. A plurality of unit cells is stacked with separators 4 having grooves 5 as gas channels. A perfluorocarbon sulfonic acid ion-exchange film is typically used as the solid polymer electrolyte. The separator 4 functions as a boundary for isolating a fuel gas from an oxidizing gas and acts as an electrical conductor between the unit cells. This requires high gas impermeability, high thermal conductivity, high mechanical strength, low electrical resistivity, low volume resistivity, and heat resistance at operation temperatures.
Such separators have been made by mechanically working artificial graphite or metallic materials, such as titanium or stainless steel. The artificial graphite separator is disadvantageous in that there is insufficient gas impermeability and it is relatively expensive, although it does have high electrical conductivity. The metallic separator is also expensive and oxidized during operation over long periods.
To solve the above problems, Japanese Unexamined Patent Application Publication Nos. 10-334927 and 11-297337 disclose separators for solid polymer-type fuel cells which are formed of compounds of artificial graphite powder and thermosetting resins and exhibit improved gas impermeability and suppressed oxidation. These separators, however, exhibit poor dimensional accuracy in the thickness direction due to orientation of particles in the planar direction. Thus, separators having high dimensional accuracy in the thickness direction cannot be stably produced. As shown in FIG. 1, each separator 4 has grooves 5 for supplying a fuel gas, such as hydrogen, and an oxidizing gas, such as air, on both faces. When the depth of the grooves 5 as gas channels is uneven, each groove has a different ventilation resistance. As a result, the fuel cell has a gas flow rate distribution which inhibits high generating efficiency.
Other important properties required for the fuel cell separator are low electrical resistivity and low volume resistivity. Japanese Unexamined Patent Application Publication No. 62-260709 discloses a separator for a phosphate-type fuel cell which is composed of graphitized meso-carbon microbeads as an aggregate and a thermosetting resin as a binder. Although this separator uses close packing of meso-carbon microbeads, conductivity is insufficient due to electrical conduction only at point contacts.
Japanese Unexamined Patent Application Publication No. 4-214072 discloses a fuel cell separator formed by hardening and firing a compound composed of carbonaceous powder, such as graphitized meso-carbon microbeads or graphite powder, and a phenol resin. This separator contains 100 parts by weight of phenol resin with respect to 50 to 150 parts by weight of carbonaceous powder. Since this separator contains such a large amount of insulating resinous component, sufficient conductivity is achieved after the hardening reaction. In order to yield high conductivity, the hardened compound is then fired to graphitize the carbonaceous powder. Graphitizing is generally performed at 2,000 to 3,000xc2x0 C. in a nonoxidizing atmosphere. Thus, industrial production of the separator by graphitizing the molded compound has problems in view of facilities, operation, and energy.
It is an object of the invention to provide a fuel cell separator having high gas impermeability, low electrical resistivity, low volume resistivity, and high dimensional accuracy, and to provide a fuel cell having high generation efficiency.
It is another object of the invention to provide a fuel cell separator having the above advantages without graphitizing the molded separator article and a fuel cell.
It is another object of the invention to provide a method for making the fuel cell separator.
According to an aspect of the invention, a fuel cell separator comprises (a) about 100 parts by weight of graphitized meso-carbon microbeads, (b) about 10 to 35 parts by weight of one of a thermosetting resin and a thermoplastic resin, and (c) about 1 to 40 parts by weight of at least one carbonaceous material selected from the group consisting of graphite powder, carbon black, and fine carbon fiber.
Preferably, the fine carbon fiber has an average diameter of not more than about 2 xcexcm and an average length of not more than about 500 xcexcm. Preferably, the graphitized meso-carbon microbeads have an average diameter of not more than about 50 xcexcm, the graphite powder has an average diameter of not more than about 10 xcexcm, and the carbon black has an average diameter of not more than about 100 nm.
The fuel cell separator is suitably used in a solid polymer-type fuel cell.
According to yet another aspect, a fuel cell includes the above fuel cell separator.
According to yet another aspect, a method for making a fuel cell separator includes mixing (a) about 100 parts by weight of graphitized meso-carbon microbeads, (b) about 10 to 35 parts by weight of one of a thermosetting resin and a thermoplastic resin, and (c) about 1 to 40 parts by weight of at least one carbonaceous material selected from the group consisting of graphite powder, carbon black, and fine carbon fiber, and molding the mixture under pressure at an elevated temperature.